The present invention relates to a method of producing a semiconductor device, and more particularly to diffusion of aluminum for forming a deep PN junction.
Among P-type impurities for silicon Si, aluminum Al has the greatest diffusion coefficient D. For instance, at 1200.degree. C., ##EQU1## where hr represents time (hours). Here, the diffusion coefficient is expressed by its square root as is usual to facilitate related calculations. This value is about 3.8 times that of boron B. For this reason, aluminum is frequently used to form a P-type region having a depth of several tens .mu.m.
To form a P-type region, aluminum is introduced into silicon crystalline structure by one of several various methods of introduction, and is then diffused by one of several various methods of diffusion. Known methods of introducing aluminum into silicon crystalline structure include the crucible method (FIG. 1A), the evaporation method (FIG. 1B) and ion-implantation (FIG. 1C). Known methods of diffusing aluminum include the closed tube diffusion method (FIG. 1A) and the vacuum diffusion method (FIG. 1D).
These methods of introducing aluminum however had various restrictions or drawbacks. With the crucible method, the vapor pressure and surface condition of aluminum vary depending on the temperature and the pressure, so that control of concentration is difficult. According to the evaporation method, aluminum is deposited on a thin-film of poly-silicon or the like which is formed on the silicon substrate. The use of the poly-silicon film is to prevent alloying of aluminum and silicon. This method is also associated with a difficulty in control of concentration. With the method of the ion-implantation, accurate control of the amount of the introduced impurity is possible, but introduction of aluminum in an amount over the solid solubility limit is not possible, so that the application of the method is restricted to low concentration diffusion (up to 10.sup.17 cm.sup.-3).
The closed tube diffusion method and the vacuum diffusion method have a disadvantage in that they require special installation and skill, and the process is complicated.